home     gallery   photography   contact   cv   videos  

HDR creation for CG lighting

The following technique is one I have used numerous times for the creation of custom HDRs. Generally I create them when I have to put CG elements (such as a camera or camcorder) into a live action scene. The HDR helps to light these CG elements with correct colour and reflections so that they feel part of the original scene.

First off you will need some equipment and software –

  • A digital SLR camera (or one that shoots RAW images).
  • Zoom lens for your camera. The longer the better.
  • A trigger for your camera. I prefer to do this remotely from a laptop to prevent any movement to the camera whilst capturing the images. I have also used a manual hand trigger when this is not practical.
  • A tripod.
  • A chrome sphere. You can buy these from specialist ball bearing manufacturers, although these can be very expensive. Alternatively get yourself a large silver Christmas decoration for a lot cheaper.
  • Some way of holding the chrome sphere secure. I use another tripod, a rod and a couple of C-clamps.
  • A free copy of HDR Shop.
  • An installation of Photoshop

Now for the technique. I will explain how I would go about creating an HDR that is to be used to help me light a camera sitting on a real table.

Firstly position your chrome ball

  • In this instance I would position it slightly above the table (around 20cm).
  • It should be held in place as firmly as possible. Ideally weight it down.
  • Using the extra tripod, rod and c-clamps I would set it up as in the diagram below. Note that I have positioned the apparatus holding the chrome ball behind the chrome ball, when looking at it from my shooting angle.

Next position your camera

  • Set up your camera on your main tripod. This should be made as sturdy as possible. Ideally weight it down to prevent it moving.
  • The camera should be placed as far away as possible from the mirrored ball, but should still be able to zoom into the mirrored ball and get it full frame (without cropping). Use the biggest zoom lens you have at full zoom. This is so that there is only a small reflection of the camera and photographer in the ball.
  • In this example I am trying to make my CG camera sit on top of a live action video plate. So I would position my stills camera so that it is facing in the same direction as the video camera that was used to shoot the live action sequence (see the diagram below).

Settings for your camera

  • You should be shooting RAW images. This is done as your camera performs post capture processing when saving as other formats which will prevent you from making a proper HDR image.
  • Put your camera in manual mode.
  • Set the ISO of your camera to a low value to prevent noise.
  • Change the aperture of your camera to it’s minimum setting. This is done to have a high depth of filed, with no depth blur.
  • Set your camera to take the photos at maximum resolution.
  • Focus on the ball and turn off auto-focus on your lens.
  • Turn off any automatic image adjustment, such as sharpening.

Shooting the photos

  • Each sequence will need to be captured over a range of shutter speeds. 10 photos per sequence should be sufficient, though if more can be done all the better. You need to cover the range from fully blown-out images to almost pitch black. Your camera should automatically add the shutter speed of the each image to its metadata, if not you will need to record this for use later.
  • If possible trigger the camera remotely from a laptop. This is to eliminate camera move when changing between shutter speeds. If this is not possible be very careful not to move the camera when changing shutter speeds.
  • In order to prevent ghosting you should have no movement in the captured environment. You may need to get people to leave the area. On a film shoot this can be a bit difficult but is necessary to get the best possible HDR image.

The next stage is compiling the HDR

Photoshop stage

  • The first step is to get all you RAW images aligned.
    • To do this open them all simultaneously in Photoshop, you will then be presented with the Camera Raw window.
    • Select all your images.
    • Set the white balance the same for all images and open them.
    • Next put all the images onto individual layers in one file. At this stage I also rename the layers the same as the original file names as this helps later on.
    • Hide all but the 2 bottom images.
    • Set the blending mode of the upper of the 2 visible layers to difference. You should now move the upper layer around to get it best aligned with the layer below.
    • Set the blending mode of the upper layer back to normal, unhide the 3rd layer up and repeat the above steps until all layers are aligned.
  • Next, crop your document so that it is just a little bit bigger than the chrome ball.
  • Save out each layer as 16bit TIF with its original name.

HDRShop stage

  • Open up HDRShop and goto – Create > Assemble HDR from Image Sequence.
  • Load in all your images.
  • You now need to tell HDRShop the relative stops between each image.
    • Make a note of all the shutter speeds and the aperture used for each shot. I usually use Adobe Bridge to read the metadata of each image for this, but there are many other programs you could use.
    • Next, using this very handy F-Stop calculator work out the stops between each of your images.
    • Plug all these values into HDRShop in the relative stops column.
  • Click ‘Generate Image’.
  • Using the + and keys brighten / darken your image until it looks like the correct brightness. You can use shift and + or for fine tuning.
  • Once you are happy with the brightness of the image goto – Image > Pixels > Scale to Current Exposure.
  • Click ‘Generate Image’.
  • Goto – Select > Draw Options > tick Circle if it is not already.
  • Click and drag your mouse over the image to make the circle. Us the adjustment handles to make it exactly the same size as the chrome ball.
  • Goto – Image > Crop.
  • Goto – Image > Panorama > Panoramic Transformations.
  • Set Source Image as mirrored ball and Destination Image as Latitude/Longitude.
  • Save your image as a .hdr file.

Use of the image

The HDR image you have created can be used in a variety of ways to light a CG scene. Here are a few examples of where I have used the method above to create an HDR for lighting –

Tags: , , , , , , , , , , , , , ,

2 Responses to “HDR creation for CG lighting”

  1. Daniel Says:

    great post, thanks for sharing

  2. arif Says:

    making is a messy thing for me, while using hdr is awesome 😀

Leave a Comment